Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, also the transmitted data volume is continuously increasing. Given the constraints of limited bandwidth for transmission, data throughput is an important aspect for users as well as network operators.
Generally, data transmission is based on the OSI layer model (OSI=Open Systems Interconnection). The highest layer (layer 7) is referred to as application layer, while the lowest layer (layer 1) is referred to as physical layer. According to OSI principles, data transmission on the application layer is independent of the physical layer actually used. The physical layer, in turn, is strongly dependent on the actual medium or channel (bearer) used. Thus, wire based transmission strongly differs from wireless transmission. However, having regard to wireless transmission, the physical layer also depends on the type of transmission medium used, i.e. infrared transmission, radio transmission, or others. Also, various different transmission protocols exist for transmission on respective OSI layers, and the transmission protocols used may depend on the data traffic to be carried.
Generally, wireless transmission in the framework of mobile data transmission or mobile communication services nowadays implies radio transmission via the so-called air interface. For this reason, also the present invention is described using radio transmission via the air interface as an example. However, the principles set out in the present application are generally also applicable to other physical channels.
Mobile communication has evolved in the past years from GSM (Global System of Mobile Communication) via UMTS/3GPP (Universal System for Mobile Communication/3rd Generation Partnership Project) up to LTE (Long Term Evolution) nowadays under discussion.
In those systems, nowadays packet switched data traffic is widely used. In packet based traffic, it is to a certain extent acceptable that some packets are lost or erroneously transmitted in transmission as the lost/erroneous data content can ignored, can be reconstructed from other packets, or, according to at least some protocols used, can be requested to be retransmitted. The loss/erroneous rate of data packets is referred to herein as packet error rate.
For example, 3GPP WCDMA HSPA (Wideband Code Divisional Multiple Access, High Speed Packet Access) and LTE systems use a mechanism of physical layer link adaptation.
Generally, link adaptation adapts a selection of modulation schemes and channel coding schemes to the currently prevailing channel conditions. This determines the data rate and/or the error probability of the link for which link adaptation is performed. Such mechanism adapts the data transmission on the physical layer (layer 1, L1) to the prevailing radio channel conditions. Typical studies of Layer 1 show that an optimum L1 capacity (also referred to as data throughput) is achieved by tuning the L1 packet error rate of the initial or first transmission to a packet error rate level of 10%. This means that there is a 10% probability that a packet is erroneously transmitted or lost. Notwithstanding such typical working assumption, it is generally of course also possible that for such initial/first transmission a packet error rate is set within the range of 5% to 30%. A typical assumption in such transmission scenario is that a transmission buffer of a transmitting device is full of data. This means that Layer 1 packets can be as large as possible and interleaving gain in time or frequency domain can be utilized together with HARQ (Hybrid Automatic Repeat Request) to reduce the error level significantly. However, if the Layer 1 packets are small (in size and/or number), it is difficult to use link adaptation and HARQ efficiently as the physical layer link adaptation and HARQ is designed for large packets.
Considering for example TCP protocol (Transmission Control Protocol) used in FTP file downloading (File Transfer Protocol) or WWW (World Wide Web) page viewing, the protocols have an impact on the buffer fill level seen at the 3GPP RLC/MAC layer or physical layer. (Note that RLC/MAC, i.e. Radio Link Control/Medium Access Control, and physical layer are tightly interconnected. Although 3GPP specifications specify link adaptation as a MAC layer functionality, for the purposes of the present application RLC/MAC and the physical layer are jointly considered to simplify the description)
In general, the offered load by the TCP traffic is scaled by TCP parameters such as round trip time and transmission window size. Considering for example FTP file downloading from network (NW) to a terminal such as an user equipment UE, this generates a relatively large amount of traffic into the downlink direction from the network, i.e. from a Node_B (according to UMTS) as a transmitter to the terminal/user equipment UE as a receiver. The direction of the data transfer in this scenario is the downlink direction. However, a small amount of traffic is also generated in reverse direction, i.e. in uplink direction. Such uplink traffic consists of for example TCP ACK packets (acknowledgement) and possible internal RRC (Radio Resource Control) control packets. Note that losing of the TCP control feedback packets is not desired because it causes retransmissions and time outs into the TCP connection.
It has been assumed earlier in some studies that CQI link adaptation should operate on the relatively high packet error rate target in order to gain from HARQ retransmissions.
Different traffic related 3GPP test configurations have been specified in other studies for WCDMA DPCH channel (Dedicated Physical Channel). It was generally assumed that 12.2 kbps AMR speech (Adaptive Multi Rate) would use 1% transport block error rate target (for a conversational QoS) and packet data service could be run at 10% physical layer transport block error rate. Data integrity would be covered by RLC (Radio Link Control) Acknowledged mode (AM). The error rate adaptation on the DPCH was made using fast power control and no CQI link adaptation or HARQ was used. Different radio bearer classes and QoS is discussed in some other documents. TCP protocol performance with ARQ (Automatic Repeat Request) was studied in still other documents.
However, there is still a need to further improve data transmission performance.